1. Field of the Invention
This invention relates to restraining apparatus and more particulary relates to a restraining mechanism which holds a pipe or conduit at a connection with another pipe, valve housing, or similar member coupled with the pipe.
2. History of the Prior Art
Various apparatus and techniques have been used for many years to couple pipe of all types with other pipe, valves, various pipe fittings, hydrants, and miscellaneous connections where pipe is secured to another member and thereafter subjected to internal fluid pressure which tends to separate the pipe from the member to which it is coupled. Typical uses for a variety of such pipe devices are sewer systems, water distribution systems, and the like. While such pipe systems have often used ductile iron pipe, it has become very common in recent years to also use plastic pipe such as pipe made from polyvinyl chloride, usually referred to as PVC pipe. It is also been common in such systems to use fittings and valves made primarily for use with cast and ductile iron pipe. The principal problem presented by such joints is gripping or grasping the pipe. Pipe joint restraining devices relying on friction alone are very unpredictable. To restrain reasonable amounts of internal pressure in pipe, it has been found necessary to create a groove in the surface of the pipe deep enough and wide enough to provide adequate shear strength to resist large axial loads created by the pressure. The soft character of PVC pipe has made it necessary to create grooves around the outer surface of the pipe over a major portion of its circumference without either damaging the pipe or reducing the capability of the pipe to withstand pressure. To create a system using PVC pipe to resist high axial loads it is necessary to provide at least one groove of sufficient depth for almost the entire circumference of the pipe. More than one groove arranged in parallel relation can also be used. To create a system to resist high axial loads without damaging the pipe it has been necessary to keep the radial loads on the pipe to a minimum and to reduce the radial loads after the groove has been formed. The mechanism used to form the restrainer connection with the pipe must include thrust resisting engaging means which remain in the groove or grooves around the pipe.
A substantial number of prior art patents show a wide variety of apparatus and methods for attachment to grip a pipe. Several devices have used an inclined plane to create enough mechanical advantage to grip a pipe to prevent separation. See the following U.S. Pat. Nos.: Gammeter, 1,898,623, Yano, 3,594,023, Sato, 3,937,500, Felker, 4,070,046, Yamaji, 4,417,754, and Hattori, 4,438,954. These devices provide no limit to the travel of the wedging member and thus there is little or no limit to the radial deflection of the pipe ring due to the extreme radial force. Also, these devices require extremely strong rings and pipe if high pressures and large diameter pipe are restrained.
Babb, U.S. Pat. No. 3,920,270 provides a front flange on his grip ring which could be used as a limit to the travel of the wedge. However, if the pressure continues to increase after the wedge reaches its limit, the result is a toggle which creates extreme radial force on the pipe as the wedge overturns.
Sato, U.S. Pat. No. 4,092,036, shows a wedging action with a limit in the form of a rear wall in a housing which contacts the wedge at the rear and thus stops the travel of the wedge. Actual experience with this device used on ductile-iron as well as PVC pipe indicates that because of the oval shaped hole in the top of the housing a toggle action around the intersection of features 28 and 43 in FIG. 7a as a pivot point allows the rear tooth to disengage and the front tooth to dive into the pipe. This action increases the radial stress in large diameter pipe at very high pressures, i.e., 500 psi.
Toggling has also been used to grip the surface of the pipe in joint restraint designs. Dillon, U.S. Pat. No. 1,930,194, Hashimoto, U.S. Pat. No. 4,647,083 and Moussiaux, British Patent 1,403,671, show toggling or Belleville spring devices. Toggling involves pivoting about a point slightly off the vertical center line of the mechanism. As the pivot point passes the vertical, deflection of the pipe or the groove usually remains. In Hashimoto, for a 12 inch pipe and a 1 inch toggle arm the maximum groove depth would be approximately 0.026 inch.
The key to a reliable restraint device for PVC pipe, especially on larger diameters, is the ability to reliably create these grooves. Other devices such as those described in Roche, U.S. Pat. No. 4,336,959 and Bradley, U.S. Pat. No. 4,568,112 form grooves in the pipe with side bolts connecting two half rings. This does not produce enough force to reliably create multiple grooves completely around a large diameter pipe such as 12 inch through 30 inch pipe. The result is slippage on the pipe and premature failure. Tests on large diameter versions of these devices have shown that grooves are created only near the side bolts. When the pressure is increased, shear failures in these limited grooves cause sudden slippage and impact on the highly stressed PVC pipe. The result is sudden premature bursting of the pipe itself at pressures well below those required by the AWWA standard.